In the scant 12 minutes devoted to foreign policy in Tuesday night’s otherwise interminable debate, moderator Jake Tapper asked the Democratic candidates whether, as president, they would declare that the United States would never be the first to use nuclear weapons in a conflict.

Only two candidates answered the question. Sen. Elizabeth Warren said she would adopt that policy. Montana Gov. Steve Bullock said he would keep the nuclear option “on the table.” Neither seemed to have thought through the issue—which is no surprise, as it hasn’t exactly been on the front burners of American politics for a while, though that in itself is a shame.

Probably few Americans realize that, from the dawn of the atomic age until now, U.S. policy has explicitly stated that we would use nuclear weapons first, if some crisis called for it—and, given that the president has the sole and unfettered power to “push the button,” he or she would be the one to make that judgment.

For the first two decades of the nuclear era, presidents had little choice. The common fear was that the Soviet army would invade West Germany or grab West Berlin, which was situated 100 miles inside East Germany. The United States didn’t have enough troops, tanks, or other conventional arms to block the offensive, so nukes were seen as the only answer—and the only deterrent—to an invasion.

President Dwight Eisenhower and his secretary of state, John Foster Dulles, called this policy “massive retaliation.” But it’s worth emphasizing that the policy called for nuclear retaliation to a conventional attack. It was assumed the United States would be the first to use nuclear weapons. In fact, for many years after, the top-secret nuclear war plans, devised by the Strategic Air Command, explicitly called for an American first strike, regardless of how the war started.

By the time John F. Kennedy took the oath of office in 1961, the Soviets had started to field their own long-range nuclear weapons. Now, if we struck them, they could strike us back. Some Western Europeans, notably French President Charles de Gaulle, wondered out loud whether an American president would “risk New York for Paris”—that is, whether he really would respond to an invasion of West Germany by nuking Moscow, knowing that the Kremlin could nuke New York City in return.

The threat of nuclear first use—the assurance that we would risk New York for Paris, or Washington for London—was at the heart of the U.S. securityguarantee.

The threat to use nuclear weapons first was so central to U.S. war planning that, during the 1961 Berlin Crisis—when the Soviets issued an ultimatum demanding that we surrender the city to Soviet control—top U.S. military commanders opposed the idea of bolstering West Germany’s defenses by sending more troops, tanks, and artillery. Their argument was that, if we were seen as preparing to fight a conventional war, the Soviets would infer that we would not use nuclear weapons first. If they believed that, they would no longer be deterred; they’d be more likely to invade.

It wasn’t just a few crazy generals who made this argument. The leaders of West Germany and other European nations—civilian and military—believed it, too. The threat of nuclear first-use—the assurance that we would risk New York for Paris, or Washington for London—lay at the heart of the U.S. security guarantee for the NATO alliance. It was—and still is—called “extended deterrence” and the “nuclear umbrella.”

In a few crises during his brief presidency, especially over Berlin, Kennedy immersed himself in this dilemma. While doing research for my forthcoming book, The Bomb: Presidents, Generals, and the Secret History of Nuclear War, I came across the minutes of a National Security Council meeting on Oct. 3, 1961, at which Kennedy noted that any use of nuclear weapons would be like “pulling the house down.”

Kennedy had been briefed—as had Eisenhower—on top-secret simulations concluding that a nuclear war with the Soviets, even if the U.S. launched first, would kill tens of millions of civilians on each side.

Yet, during this phase of the Cold War, defending Berlin was central to defending the free world; almost no one disputed that. And so, Kennedy continued to act and speak, in public, as if he would start a nuclear war—as if he would, in effect, commit mass murder and national suicide.

After two close calls, first in Berlin, then Cuba—crises in which war was avoided by a mix of force, diplomacy, and luck—Kennedy saw that the only way out of this dilemma was to end the Cold War itself. He and Soviet Premier Nikita Khrushchev began to take promising steps toward that goal, until Kennedy was assassinated in November 1963 and Khrushchev was ousted by hard-liners about a year later.

The arms race began in full force at that point. For the next 35 years, a few presidents, notably Richard Nixon, sometimes threatened to use nuclear weapons, but none came close to ordering their use. And yet, neither did any president even entertain the idea of abandoning nuclear first use as official U.S policy.

President Barack Obama tried to break that barrier. He entered the White House pledging to “reduce the role of nuclear weapons” in U.S. security. One way to do that, in the minds of many arms-control advocates (some of whom were working on his staff), was to declare a policy of no first use.

In the fall and winter of 2009, Obama held a few behind-closed-door meetings to debate the idea. Those in favor argued that a first-use policy was no longer necessary. The Cold War was over, the Soviet Union was gone, the new Russia (still fairly supine and eager for cooperation with the West at that point) wasn’t going to invade Europe; and if some conflict did take place, the new generation of U.S. high-tech conventional weapons—GPS-guided drones and smart bombs, which exploded with pinpoint accuracy—could destroy targets that only nukes could have leveled in decades past. Finally, proponents argued, if the U.S. declared that it no longer found the bomb useful except as a second-strike deterrent, maybe other countries would steer clear of the nuclear genie.

Top officials in the State Department and the Pentagon opposed the idea. So did U.S. allies, especially those in Asia, who still counted on the nuclear umbrella as the ultimate security guarantee. (When news leaked that Obama was just considering a no-first-use policy, Japan swiftly sent emissaries to Washington to plead the case against the idea.) Secretary of Defense Robert Gates also raised the threat of a massive biological attack. The U.S. had long ago disposed of its own bioweapons, so it couldn’t “retaliate in kind” if another country struck first. Certainly, Gates argued, a president would at least want to consider responding to such a hideous attack with nuclear weapons. Therefore, we should make that possibility clear to all potential aggressors from the outset—in order to deter them from contemplating an attack. Why, he asked, should we box ourselves in unilaterally?

According to several officials who took part in these meetings, Obama thought—and said—that no president in his right mind would use nuclear weapons first. However, he also thought Gates’ arguments had merit; he had no desire to make Japan and other allies nervous about America’s commitments; and, to the surprise of some of his aides (and the relief of the generals), he, like Gates, saw no point in unilateral gestures.

At the same time, he wanted to make it clear—to his own people and to the rest of the world—that the U.S. would use nukes first under only extremely limited circumstances. He came up with this formula: The United States would not use nuclear weapons first against countries that (a) did not possess nuclear weapons and (b) had signed, and were abiding by, the Non-Proliferation Treaty. This preserved the option of going first against what many considered the main threats—Russia, China, North Korea, and (if it ever developed a bomb, as it seemed to be doing at the time) Iran.

A coda to the Obama years: In late 2016, some of his White House aides wrote a speech—which they hoped he might deliver as a farewell address—saying that no first use was a sensible idea after all and that, as the speech put it, the “sole purpose” of nuclear weapons was to retaliate against another country’s nuclear strike. Obama passed on the speech, but he let Vice President Joe Biden deliver it on Jan. 11, 2017, nine days before their term ended.

In the first round of Democratic debates, one month ago, Biden repeated this point—that he and Obama had concluded second-strike deterrence was the “sole purpose” of nuclear weapons. However, he added that he would make no-first-use U.S. policy only in consultation with senior military officers and allies—which means he isn’t likely to make it policy.

This has long been the dilemma in deterrence: Nuclear war is an insane option, but you have to make adversaries believe you’d actually push the button, in order to keep them from getting too aggressive. The only real way out of this trap is a transformation of global politics—and that doesn’t seem in the offing.

In other words, Warren was right that it’s never a good idea to use nuclear weapons.

But she seemed unaware of the long history of this debate—or maybe the absurd one-minute limit on all answers precluded her from tracing its complexities—when she said that declaring a no-first-use policy would “make the world safer.”

America’s allies have a different view. And to the extent she has a point, it’s worth noting that the Russian military now has a doctrine of using nuclear weapons first if NATO troops make incursions on Russian territory—mainly as a way of countering America’s supremacy in conventional arms.

And so the world has come full circle from the days, a half-century ago, when the United States declared it would use nuclear weapons first as a counter to Russia’s superiority in conventional arms. There’s a reason why the theory of mutual assured destruction—if you nuke us, we’ll nuke you—has long been abbreviated as MAD.

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